The invention relates to a piston-cylinder unit having a sealed pressure piston, preferably for extrusion cartridges for liquids having a higher viscosity. Such piston-cylinder units are used in hydraulic systems, control and slide valve rods, pressure generators or extrusion units, such as, for example, caulking gun-operated extrusion cartridges for pasty putties or for liquids such as, for example, colored lacquers. The invention is used, in particular, with ready-to-use cartridges in which the piston is formed by the bottom of the pressing closure.
Particularly in the above-mentioned extrusion devices, the pressure pistons have a pressure build-up effect only in one direction so that a seal against the inner wall of, for example, a cylindrical extrusion unit or cartridge, as the case may be, need be effective in only one direction of movement. The cartridges are designed as piston-cylinder units constituted by slender cylindrical vessels which have a fixed bottom provided with an openable discharge opening, preferably shaped as an extrusion nozzle, and a bottom closure which is axially movable within the cylinder and acts as a pressure piston. Thus, the bottom closure can be moved in the direction of the medium in the cartridge by means of an attachable pressure plunger of the caulking gun for expelling the medium through a discharge opening by compression. Seals are known for this purpose which employ sealing cuffs similar to those on an air pump valve or seals formed as annular circumferential collars along the outer wall of the cylindrical pressure piston.
Particularly in the field of ready-to-use cartridges, the cylindrical cartridges as well as the pressure pistons inserted into the cartridges are made of plastic or plastic-like materials, which become flowable under a permanent compression stress even at normal temperatures and are partially plastically deformed. Thus high quality sealing means which act with greater sealing forces on the inner walls of the cartridge can usually not be used. Since the storage period of such cartridges may, under certain circumstances, extend over several months before use, the pressure forces would act on the same portions of the walls during this storage period and could produce undesirable deformations. For that reason, the sealing systems for this field of application are designed so that the sealing forces remain correspondingly small in order to avoid deformations.
Because of the above circumstances, however, failure of the seal between the piston wall and the cartridge result already during storage or under a light permanent pressure on the bottom of the piston. This leads to leakages even before use, resulting in losses of some of the material and in soiling of the processing devices.
As already mentioned, it is known to use sealing cuffs or piston ring seals of the shaft sealing ring type.